About 30 percent of children with attention-deficit hyperactivity disorder do not respond to prescription medication.

Sometimes, they turn to natural remedies, such as St. John's wort. But a new study in JAMA says that the plant does not easy symptoms.

Wendy Weber of Bastyr University studied 54 children from ages 6 to 17 with ADHD. Half were given St. John's Wort each day for eight weeks, and the other half took a placebo.

"We found that the individuals who took St. John's wort did no better than the children who took placebo. So we saw no beneficial effect of St. John's wort in our trial," she said.

Symptoms such as restlessness, hyperactivity and lack of concentration persisted, she said.

"Even though it's a natural product does not necessarily mean that it's safe, and so you always want to talk to your healthcare provider about using those natural treatments and keep them informed because there can be interactions," she said.

In fact, St. John's wort can limit the effectiveness of other drugs.

"It enhances the function of the liver, and by doing that it increases the clearance of other medications, and so when someone adds St. John's Wort to their existing medication it can make that other medication not work," she said.

Reports on the research did not address if the study compared side effects from St. John's wort to prescription medications that are used to treat ADHD.